"The horror of 9/11 meant we briefly lost sight of the standards and rule of law that took centuries to develop and fundamentally distinguish us from the terrorist."

He added: "Given the scale of foreign fighters we should consider an agreed international process involving The Hague, which ensures terrorists from any origin are transparently and fairly held account for their actions."

The families of some of the cell's victims have also said the two captured fighters should face trial.

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The US Department of Defence spokesman said they were "still considering options".

"Rest assured our intention is to hold anyone accountable who commits acts like those they are alleged to have committed," Major Adrian Rankine-Galloway from the department said.

He confirmed the captured fighters were being held by US-backed Syrian Democratic Forces in a "detention location" in Syria.